Even from the comparatively small amount of work that has been done in this area it is already clear that expert systems can be of value in many architectural applications. This is particularly so in those applications involving what broadly can be called, 'classification' (such as fault diagnosis, testing for conformity with regulations and so on). What we want to look at in this chapter are some of the developments in knowledge-based systems (KBS) which will be needed in order to make them more useful in a broader application area and, especially, in creative design. At the heart of these developments will be two things: (1), more appropriate methods of representing knowledge which are as accessible to humans as they are to computers; and (2), better ways of ensuring that this knowledge can be brought to bear exactly where and when it is needed. Knowledge engineers usually call these elements, respectively, 'knowledge representation' and 'control'.